1. Field of the Invention
The present invention relates to a display control device, and more particularly to a display control device adapted for use in a display device employing a display element exhibiting bistability to the electric field, such as ferroelectric liquid crystal display device.
2. Related Background Art
Among display devices employing liquid crystal compound, there is already known a device in which a group of scanning electrodes and a group of signal electrodes are positioned in the form of a matrix and a liquid crystal compound is filled therebetween for forming plural pixels thereby displaying image information.
For such a display device there has been employed so-called time-division driving method in which voltage signals are cyclically applied to the scanning electrodes and information signals are applied in parallel manner to the signal electrodes, in synchronization with the voltage signals supplied to the scanning electrodes. Such a display device and such driving method are associated with a difficulty in increasing the density of the pixels, or in increasing the image size.
In various liquid crystal compounds, there has almost solely been employed, in the display devices, the twisted nematic (TN) liquid crystal because of a relatively high response speed and a low power consumption. In the liquid crystal of this type, the nematic liquid crystal molecule with positive dielectric anitropy assumes a twisted (helical) structure in the absence of electric field, as shown in FIG. 50A, in the direction of thickness of the liquid crystal layer, and the liquid crystal molecules constitute, between the electrodes, twisted structures which are parallel mutually and in each layer. On the other hand, under an electric field, as shown in FIG. 50B, the nematic liquid crystal molecules with positive dielectric anisotropy are aligned in the direction of electric field, thus inducing optical modulation. If a display device is formed by employing such liquid crystal in combination with electrodes of a matrix structure, in an area where a scanning electrode and a signal electrode are both selected (selected point), there is applied a voltage exceeding the threshold value required for orienting the liquid crystal molecules perpendicularly to the electrodes, but, in an area where both of the scanning electrode and the signal electrode are not selected (unselected point), the above-mentioned voltage is not applied so that the liquid crystal molecules retain the twisted stable orientation parallel to the electrodes. By placing linear polarizers in mutually crossing relationship on both sides of such liquid crystal cell, the light is intercepted in the selected point but is transmitted in the unselected point because of the light-rotating property of the twisted structure of the liquid crystal. In this manner an image display device can be obtained.
However, in such a matrix electrode structure, a certain electric field is applied also to so-called half-selected point where the scanning electrode is selected but the signal electrode is not selected, or, the scanning electrode is not selected but the signal electrode is selected. The display device functions in normal manner as long as the voltage supplied to the selected point is sufficiently different from that supplied to the half-selected point, and the threshold voltage required for orienting the liquid crystal molecules perpendicularly to the electrodes is present between the above-mentioned voltages.
However, when the number N of the scanning lines is increased in such structure, the duty ratio, or the ratio of duration of effective electric field on a selected point to the period of scanning of the entire frame decreases as 1/N. Consequently the voltage difference, in the effective value, between the selected point and the unselected point in the repeated scanning operations decreases with the increase in the number of scanning lines, thus giving rise to lowered image contrast and crosstalk.
Such phenomena are fundamentally unavoidable in driving conventional liquid crystal lacking bistability (in which liquid crystal molecules are stable when oriented parallel to the electrodes and are perpendicularly oriented only during effective application of the electric field) by means of time accumulating effect (namely by repetitive scanning). In order to overcome such difficulties there have been proposed various methods such as voltage averaging method, two-frequency driving method, multiple matrix method etc., but these methods are still insufficient and the image size and the pixel density of the display devices have been limited by the limitation in the number of scanning lines.
Also for overcoming the above-mentioned drawbacks, the present applicant has already proposed driving methods for liquid crystal exhibiting bistability to the electric field, for example in the U.S. Pat. No. 4,655,561 issued on Apr. 7, 1987. For use in such driving methods, there is preferred chiral smectic liquid crystal with ferroelectricity, particularly that of C-phase (SmC* or H-phase (SmH*).
In the SmC* phase, as shown in FIG. 51, the liquid crystal molecules have parallel layered structure, in which the longer axis of the molecule is inclined to the layer. The molecules constitute a spiral structure as the direction of inclination of molecules is different amount different layers.
In the SmH* phase, as shown in FIG. 52, the molecules show parallel layered structure, with an inclination of the longer axis of the molecule to the layer, exhibiting a hexagonal packed structure in a plane perpendicular to the longer axis of the molecule.
In the SmC* or SmH* phase, the liquid crystal molecules assume a spiral structure, as schematically shown in FIG. 53.
In FIG. 53, e3 indicates a liquid crystal molecule; e4 an electric dipole moment; and e5 a boundary of layers. Each liquid crystal molecule e3 has a dipole moment in a direction perpendicular to the longer axis of the molecule, and moves with a fixed angle .theta. to the Z-axis perpendicular to the boundary plane e5 of the layers, thus constituting a spiral structure. The illustrated state exists in the absence of an applied voltage, but, in the presence of a voltage exceeding a certain threshold value in the direction of X-axis, the liquid crystal molecule is oriented in such a manner that the electric dipole moment e4 becomes parallel to the X-axis.
As the SmC* or SmH* phase can be realized in the course of phase transition by temperature, it is desirable, in case of using such liquid crystal compound, to select the display device according to the temperature range of use of the display device.
FIG. 54 schematically illustrates a cell utilizing the ferroelectric liquid crystal (FLC) explained above. Substrates (glass plates)el, el' respectively have transparent electrodes composed for example of In.sub.2 O.sub.2, SnO.sub.2 or indium tin oxide (ITO), and the liquid crystal of SmC* phase is sealed therebetween in such a manner that the layers e2 of the liquid crystal molecules become perpendicular to the glass plate surfaces. The liquid crystal molecule e3, represented by a thick line, has a dipole moment e4 in a direction perpendicular thereto. When a voltage, exceeding a fixed threshold value, is applied between the electrodes of the substrates el and el', the spiral structure of the liquid crystal molecule e3 is unwound and the orientation of the molecules e3 is changed in such a manner that the dipole moments e4 are all aligned in the direction of electric field. Because of the oblong shape, the liquid crystal molecule e3 shows anisotropy in the refractive index between the longer and shorter axis. It will therefore be easily understood that a liquid crystal optical modulating device in which the optical properties vary according to the polarity of applied voltage can be obtained by placing mutually crossing polarizers on both sides of the glass plates.
If the liquid crystal cell is made sufficiently thin (for example 1 .mu.m), the spiral structure of the liquid crystal molecule becomes unwound even in the absence of the electric field, as shown in FIG. 55, and the dipole moment p or p' thereof assumes an upward or downward position. If an electric field E or E' exceeding a threshold value is applied for a predetermined period, as shown in FIG. 55, the dipole moment is changed upwards or downwards according to the field vector of the electric field E or E', and the liquid crystal molecules are correspondingly oriented in a first stable state f3 or a second stable state f3'.
The use of such ferroelectric liquid crystal in the optical modulating device provides following two advantages: first, a very high response speed (1 .mu.sec-100 .mu.sec), and, second, bistable nature of the orientation of the liquid crystal molecules.
The above-mentioned second advantage will be further explained with reference to FIG. 55. Under the application of an electric field E, the liquid crystal molecules e3 are oriented in the first stable state f3, which remains stable even after the application of the electric field is discontinued. Under the application of the inverse electric field E', the liquid crystal molecules e3 are reoriented into the second stable state f3', which again remains stable even after the application of the electric field is terminated. Thus the liquid crystal molecules have a memory property, and retain their oriented state unless the applied electric field exceeds a certain threshold value.
In order to effectively exploit such high response speed and memory property, the cell is preferably as thin as possible, generally in a range of 0.5 to 20 .mu.m, particularly 1 to 5 .mu.m.
Now reference is made to FIGS. 47 to 49D for outlining the driving method for the ferroelectric liquid crystal.
FIG. 56 is a schematic view of a cell having matrix electrodes, between which a ferroelectric liquid crystal compound (not shogun) is sandwiched. There are illustrated common scanning electrodes com and signal electrodes sig. At first there will be explained a case in which a scanning electrode com1 is selected.
FIGS. 57A and 57B respectively show an example of an electrical scanning signal supplied to the selected scanning electrode com1 and an electrical scanning signal supplied to other (unselected) scanning electrodes com2, com3, com4, . . . FIGS. 57C and 57D respectively show an example of an electrical information signal supplied to the selected signal electrodes seg1, seg3, seg5 and an electrical information signal supplied to other unselected signal electrodes seg2, seg4.
In FIGS. 57A-57D and 58A-58D, the ordinates indicates voltage while the abscissa indicates time. For example, in case of displaying a moving image, the scanning electrodes com are cyclically selected in succession. It is assumed that, in a liquid crystal cell showing histability for a predetermined voltage duration .DELTA.t.sub.1 or .DELTA.t.sub.2, a threshold voltage -V.sub.th1 is required for realizing the first stable state and a threshold value +V.sub.th2 is required for realizing the second stable state. The voltage supplied to the selected scanning electrode coml is, for example, as shown in FIG. 57A, an alternating voltage which is 2 V for a phase (duration) .DELTA.t.sub.1 and -2 V for another phase (duration) .DELTA.t.sub.2. The application of such electrical signal having different voltages in plural phases to the selected scanning electrode can cause rapid state changes between first and second stable states corresponding to optically dark and light states.
On the other hand, the other scanning electrodes com2, . . . , com5, . . . are given, as shown in FIG. 57B, a central potential of the voltages supplied to the cell, namely a reference potential (for example ground potential). The selected signal electrodes seg1, seg3, seg5 are given an electrical signal V as shown in FIG. 57C, while the unselected signal electrodes seg2, seg4 are given an electrical signal -V as shown in FIG. 57D. The above-mentioned voltages are suitably selected so as to satisfy the following relations:
V&lt;V.sub.th2 &lt;3 V
-3 V&lt;-V.sub.th1 &lt;-V
FIGS. 58A and 58B respectively shown the voltages supplied to the pixels A and B show the FIG. 56. As will be apparent from these charts, the pixel A, positioned on the selected scanning line, receives a voltage 3 V exceeding the threshold value V.sub.th2 in a phase .DELTA.t.sub.2. Also the pixel B, positioned on the same scanning line, receives a voltage -3 V exceeding the threshold value -V.sub.th1 in a phase .DELTA.t.sub.1. Therefore, on the selected scanning line, the liquid crystal molecules are oriented in the first or second stable state, respectively according to whether the signal electrode is selected or not.
On the other hand, as shown in FIGS. 58C and 58D, on the unselected scanning line, each pixel receives a voltage V or -V, which does not exceed the threshold values. Therefore, in each pixel which is not on the selected scanning line, the molecules retain an orientation corresponding to the signal state in the preceding scanning operation. In this manner signal of a line are written when a corresponding scanning electrode is selected, and the written signal states are retained until the succeeding selection in the next frame. Consequently the effective duration of selection per line remains same even when the number of scanning electrodes is increased, so that image contrast is not affected.
As explained in the foregoing, there have been made proposals on the ferroelectric liquid crystals in order to realize a display device exhibiting bistability to the electric field and capable of retaining the stable state even in the absence of electric field, thereby overcoming the difficulties associated with the conventional display devices relying on the twisted nematic liquid crystal, but there still remain various issues to be considered on the driving method of such display device utilizing the ferroelectric liquid crystal.